This invention relates generally to transducers and transducer arrays and, more particularly, to ultrasonic transducer arrays such as those used in medical imaging. Various aspects of the invention also relate to a method of manufacturing apodized transducers.
A transducer converts energy from one form into another form (for example, from mechanical energy to electrical energy or vice versa). Transducers in audio loudspeakers, for example, convert electrical signals into mechanical vibrations that in turn create audible sound waves. Similarly, transducers are often used to generate high frequency ultrasonic waves for various applications such as medical imaging, non-destructive evaluation (NDE), fluid flow sensing, non-invasive surgery, dentistry and the like. Transducers are widely used in the field of medicine for investigative purposes. For example, an ultrasound transducer makes it possible to observe the development of a baby in its mother""s womb. This non-intrusive procedure assists doctors in estimating the date that the child will be born, and in verifying the proper development of the baby by noting, for example, details as tiny as the four chambers of the heart and the development of the lungs. This medical advance is facilitated by ultrasonic sound waves which are transmitted by the transducer and which are variably reflected off of varying types of tissue inside the body. The transducer receives these reflected ultrasonic signals and converts these ultrasonic signals into electrical signals which can be used to generate, for example, a two-dimensional picture of a baby or organs within the human body.
Ultrasonic technology has made large technological advances in recent years. For example, one kind of transducer that has experienced technological advances is a Brightness mode transducer (B-Mode). In a B-mode transducer, the amplitude of reflected pulses (i.e. the strength of a reflected ultrasonic signal) is indicated by the brightness of a dot. By scanning an entire area of interest, multiple dots are combined to map out an image for display. The area of interest can be scanned, for example, by moving the transducer linearly or in an arc like motion. Until the 1970xe2x80x2s, virtually all B-mode imaging systems required several seconds to produce an image. Consequently, these systems were limited to imaging non-moving targets. Since that time, rapid two-dimensional B-mode imaging, known as xe2x80x9creal-time scanningxe2x80x9d, has enabled visualization of moving targets within the body. In order to create a useful display of the moving targets within the body, methods were developed to rapidly move the acoustic beam throughout the area of interest inside the body. Three primary methods have been developed to rapidly move the acoustic beam: mechanical sector scanners, sequential linear arrays, and phased linear arrays. Mechanical sector scanners rapidly move the acoustic beam using one or more piston transducers which may be rocked or rotated about a fixed axis with, for example, an electric motor. Linear arrays generally consist-of a number of small individual transducers arranged side-by-side in a single assembly. Sequential linear arrays typically produce two-dimensional images in a rectangular format by transmitting on each of the array elements (or small groups of elements) and receiving the echo information with the same elements. Phased array scanners are the most sophisticated real-time systems. Phased array systems produce images by rapidly steering the acoustic beam through the target by electronic rather than mechanical means. The phased array scanners produce the pie-shaped image commonly seen in medical ultrasound applications, and popularly known as the xe2x80x9csector-scanxe2x80x9d. These three systems have been generally described by Somer and Von Ramm.
Obviously, the ability to have a high quality resolution is important to producing accurate and readable images. There are three aspects of resolution which are relevant to ultrasound imaging: spatial resolution, contrast resolution, and temporal resolution. Spacial resolution generally refers to the ability to distinguish registrations in the displayed image of objects that are close together. Contrast resolution generally refers to the ability to produce distinguishable differences in the brightness of two different types of materials which would have slightly different echogenicities. For example, a tendon might reflect at a different brightness than a muscle. Temporal resolution refers to the ability to display an image when the object being imaged is moving.
One of the factors that interferes with achieving high resolution in these areas is the fact that the ultrasound signal undergoes attenuation and dispersion as it progresses deeper into tissue. This degradation is governed by the Kramer-Kronig relationships. See, M. O""Donnel, E. T. Jaynes, and J. G. Miller, Kramer-Kronig, Relationship Between Ultrasonic Attenuation And Phase Velocity, J. Acoust. Soc. Am. 69(3), March, 1981, pp. 696-701. One method of improving resolution is to frequency apodize the transducer aperture. A previous attempt to achieve this frequency apodization is described by U.S. Pat. No. 5,902,242. In this patent, the central zone of the array element is thin (elevation direction) and gradually thickens nearer the edges of the aperture. Two ultrasonic images are created using a first relatively high ultrasonic imaging bandwidth transmit pulse and a second narrower bandwidth transmit pulse. The first pulse activates the full aperture and creates an image that has relatively high axial resolution and relatively low elevational resolution. The second pulse activates the narrower width portion of the aperture and creates an image that has relatively lower axial resolution and a higher elevational resolution at ranges spaced from the geometric focus. Combining these two frames yields an image which has both enhanced spatial and contrast resolution. This method, however, offers some significant manufacturing challenges. The general functionality disclosed in U.S. Pat. Nos. 5,902,242 and 5,479,926 are incorporated herein by reference.
Another factor that interferes with achieving higher resolution is the existence of xe2x80x9cside lobesxe2x80x9d in the ultrasonic beam. When an ultrasonic beam passes through a human body or other medium, xe2x80x9cblurringxe2x80x9d occurs as the beam is defracted (i.e. bent) creating side portions (i.e. xe2x80x9cside lobesxe2x80x9d) which accompany the desired main lobe of the ultrasonic beam. The side lobes act as interference and tend to degrade the ability to achieve high resolution. Past attempts have been made to suppress the side lobes. One conventional method of suppressing side lobes is to apply an amplitude apodization function to the electrical signal, usually a Gaussian or Hanning function, to shape the electrical signals received by the array. (See, for example, Apodization of Ultrasound Transmission, U.S. Pat. No. 4,841,492 incorporated herein by reference.) An apodization function is applied to smoothly taper down to zero the edges of a sampled region of a signal. This electrical signal apodization has several undesirable aspects. For example, while in-plane (azimuth direction) electrical signal apodization is possible, out-of-plane (elevation direction) electrical signal apodization may not be possible in a 1D arrays because one signal connects across the whole elevation aperture. Although out-of-plane electrical signal apodization could possibly be done for 2D arrays, where the elevation aperture is discretized and can be electrically addressed individually, this may be quite difficult to achieve due to the electrical complexity, muxing, etc.
Another method of achieving amplitude apodization is to place a thin sheet of acoustic blocking layer over the front surface of the transducer to substantially block the ultrasonic wave emission from a portion of the front surface area, thus defining an inactive area. (See, generally, Ultrasonic Transducer Apodization Using Acoustic Blocking Layer, U.S. Pat. No. 5,285,789, incorporated herein by reference.) This approach generally removes the edges of the transducer from operation and has the effect of suppressing the side lobes. There are at least two problems with this approach: the first is that an extra layer is typically added to the transducer stack making manufacture more difficult, and the second is a loss of sensitivity due to blocking of the aperture (reducing the strength of the signal which can be converted to an electrical signal).
Another method of achieving side lobe suppression is to apply different levels of polarization across the transducer elevation. In this way, segments of the transducer near the center are polarized much more strongly than other transducer elements near the edges of the transducer. This has the effect of suppressing the side lobes at the outer edges and transmitting somewhat amplified main lobe signals. The disadvantage of this method is that it is typically difficult to manufacture. Polarization generally requires a difficult process of applying a voltage across each individual transducer element. Because this process is typically very sensitive, breakage is more likely to occur, resulting in ruining the transducer.
Another method of suppressing side lobes involves the construction of a transducer from individual piezoelectric ceramic rods which are positioned so as to create a mechanical apodization. This is typically done by placing more ceramic rods near the center of the transducer than near the edges of the transducer. More of the sound waves, therefore, are transmitted near the center and thus the main lobe is transmitted and the side lobes are suppressed. (See Piezoelectric Apodized Ultrasound Transducers, U.S. Pat. No. 4,518,889 incorporated herein by reference.) The major disadvantage of this method is that manufacturing of such a transducer is typically extremely difficult. It can be difficult to individually place the ceramic rods in an inert binder and, furthermore, it is generally difficult to make electrical connections to the back of each individual ceramic rod. This drastically increases the chances of breaking one or more rods and destroying the transducer.
Another method of suppressing side lobes involves select removal of the metalization electrode from the outer edges of each element of the piezoelectric material. In effect, the piezoelectric rods exist in an even pattern but some of them are not connected or are only weakly connected. This creates an apodization attenuation function which modifies the ultrasound beam in the elevation plane. (See Ultrasonic Transducer Array With Apodized Elevation Focus, U.S. Pat. No. 5,511,550, incorporated herein by reference.) The disadvantage of this method is that selective removal of the metalization electrode from the transducer leaves discreet boundaries between the metalization and non-metalization areas which causes undesirable edge effects in the electric field density.
Therefore, despite all the attempts to create an improved ultrasound resolution through frequency and amplitude apodization, there still remains a need for a way to manufacture an ultrasound transducer with frequency and/or amplitude apodization capabilities which nonetheless does not involve the difficulties of manufacturing mentioned above.
A new method and apparatus for apodization is exemplified in an ultrasound transducer used, for example, in medical applications. Various embodiments of the method and apparatus enable the ultrasound transducer apparatus to modulate the resonance frequency across the aperture or suppress side lobes; thus improving signal quality and making it possible to produce improved images. The manufacture of this apparatus may be improved by the making of composite cuts in the piezoelectric material according to a specific pattern which generally provides a lesser/greater concentration of piezoelectric material near the middle of the transducer and more/less material near the edges of the transducer or vise versa. Concentration of piezoelectric material can be varied across the surface of the piezoelectric transducer by varying the spacing between the cuts in the piezoelectric material, or by varying the width of the cuts in the piezoelectric material, or a combination of both. Increasing the concentration of piezoelectric material near the edges as compared to the center effectively frequency modulates the ultrasound signal across the aperture. In this embodiment, some amplitude apodization may also be achieved. Alternatively, reducing the concentration of piezoelectric material near the edges of the transducer in comparison to the center effectively lessens the impact of side lobe signals. Therefore, by varying the size of composite cuts, and/or the spacing between the cuts, frequency and/or amplitude apodization may be achieved, improving signal quality, while maintaining a simple method of manufacturing.